Chemical removal of rust



United States Patent Ofiice Patented July 18, 1961 2,992,945 CHEMICAL REMOVAL OF RUST Isidore Geld, 56-11 184th St., Flushing 65, N .Y., Walter L. Miller, 160 Hendrickson Ave., Lynbrook, N.Y., and gidneyY Tudor, '102--17 64th Road, Forest Hills 5, N. 3 No Drawing. Filed Aug. 12, 1957, Ser. No. 677,807 a 17 Claims. (Cl. 134-22) (Granted under Title 35, U.S. Code (1952), see. 266) i The invention described herein may be manufactured and used by and for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention concerns chemical removal of rust from ferrous metals and particularly concerns removal of rust from iron and steel.

When iron, steel or other ferrous metal is exposed to the oxidizing influence of moist air or certain chemicals, there accumulates upon the exposed surface a coating of rust. A characteristic coating of rust includes a layer of ferrous oxide binding the rust to the metal surface and a porous layer of other iron oxide overlying the binding layer. For example, a coating of rust typically is formed of a binding layer of ferrous oxide, an intermediate layer of magnetite and an outer layer of ferric oxide. In certain industrial and commercial uses the coating of rust under aggravated conditions or over a long period of time builds up to a considerable thickness. Such rust formations in some cases interfere with industrial and commercial use and must be removed.

Thus, for example, the inner surfaces of a fuel cargo tank aboard a tanker ship will in use become coated with rust. Because the rust is a porous mass, it absorbs salt, water, and various impurities which contact it at intervals with the result that when the tank is filled the cargo becomescontaminated with salt, water, and various impurities that migrate out from the rust pores. This contamination tends to degrade the cargo and requires con siderable processing to restore the purity. When contamin-ation from rust scale impurities become severe it is necessary to remove the rust, and in some instances to' replace it with a paint coating. In such cases the innor tank surfaces must be completely derusted in such manner that the steel surface beneath the. rust be not significantly roughened or perforated.

A commonly used method for removing rust from inner surfaces of large tanks is sand blasting which is costly, time consuming, and requires extensive labor and specialized equipment. Other conventional methods for removing the rust include wire brushing and flame cleaning, both of which fail to remove heavy rust completely. The chemical removal of rust or scale commonly termed as pickling, had not been considered as suitable for large tanks, particularly on tanker ships. Conventional pickling methods include the use of hot sulfuric acid, usually 2% to 10% by volume, or cold hydrochloric acid, 10% to 50% by volume of the concentrated acid, or combinations of thesetwo- A relatively low concentration of one-half percent sulfuric acid is used but only near boiling temperature as specified in U.S. Patent 1,434,011. All of these pickling methods are well suited for their purpose which is to remove rust or mill scale from ferrous surfaces wi-thin a brief time period. None of these pickling methods is suitable under the necessary conditions for use on a tanker ship. Such conditions require at least severalQhoursto fill and empty cargo tanks. Because of the very large volume it is impractical to neutralize the used acid for disposal and it is necessary to dispose of it outside harbor limits by means of the usual cargo pumps. The standing period may well require more than 2 in completely removing the rust are unsatisfactory in that they will cause severe damage to tank structures, ship piping and cargo pumps within one day. Aluminum ladders as used in some tanks would be severely attacked in methods of the prior art. I

We have discovered a new method of derusting by acid pickling that is in sharp departure from the prior art, and whereby rust, including exceedingly thick formations, such as in the order of M4 inch, can be completely re moved without significant chemical attack of the surface upon which the rust is formed. According to our 'discovery, unexpectedly we are able to remove rust by soaking with cold dilute acid, such as sulfuric acid. We have discovered how to combine variable factors including acid concentration, temperature and time span of soaking so that the activity of the acid is insuflicient to attack the surface on which the rust is formed but is sufficient to remove the rust completely.

More particularly we have discovered a new manner of removing an oxidized coating from ferrous metals, especially removing rust from iron and steel, by a chemical action which does not attack significantly the surface upon which the rust is formed.

Our discovery is particularly applicable to the chemical removal of rust from the inner surfaces of cargo tanks on tanker ships bysoaking with cold dilute acid, such as sulfuric acid, and determining variable factors including concentration, temperature and time of soaking so that the activity of the acid is insufiicient to attack the surface on which the rust is formed or to attack incidental nonferrous surfaces but is sufiicient to remove the rust completely.

Many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description.

In removing rust in accordance with our discovery the rusted surface is first cleared of any material in contact therewith so that the rust is completely exposed. In the case of a cargo or storage tank this is accomplished by emptying the-tank. A detergent or solvent rinse may be required only if non-volatile oily contaminants remain, such as heavy or crude oils. There is then prepared a cold dilute pickling bath of acid such as sulfuric acid. In the case of a cargo or storage tank the pickling bath can be prepared in situ in the tank itself, for example, by

' partly filling the tank with water, then adding sulfuric acid, then adding more water until the tank is full. The pickling bath is neither heated nor cooled, but allowed to become thermally stable at the temperature of the ambient atmosphere or tank. Optionally, the bath may include an inhibitor such as a halide salt for inhibiting attack of the base metal on which the rust is formed and to this end sea water can be used in place of fresh water in forming the bath.

The rusted surface is then soaked in the pickling bath,

and in the case of a tank with the bath formed in situ one full day. All prior art methods that are successful f in the tank, such soaking occursas the tank becomes filled to the desired level and remains filled. The time span of treatment is determined by factors including the nature of the rust and in any case the time span is made adequate to remove all of the rust. After the rust is completely removed the newly cleaned surface is removed from the pickling bath and rinsed, and in the caseof a tank this is accomplished by emptying the tank and then rinsing the inside thereof.

Example I The following derusting method was carried out on a' cargo tank of a United States Navy tanker ship, where the tanks were heavily rusted with a scale approxim'ately A inch thick. 1

(a) The tank was first about /3 filled with sea water.

(b) Concentrated sulfuric acid, 66 Baurn, was added in the amount of 0.28 percent by volume on the hasi ..a..-full.tank.

- c) Thetank 'was further. filledfwithseaw'at'er; to.ju st filqwnthe tank. top. Theconcentration was determined by. analyzing the. sulfuric acidcontent at the bottom, middie and top' of the tank. Filling, of. the tank with sea water was then completed. The concentration was checked againafter 24 hours,

(d), This acid solution was. left in the tank, so that the rust was allowedto soak for 3 days. and the temperature was, approximately 63 F.

(e) Periodic tests were made of acid strength and it was found thatthe strength remained substantially constant during the first part o'f-the, soak but that the acid depleted rapidly to a lower concentration at the end of the soak.

(f) At the end of the soak the scale and rust had fallen to the bottom of the. tank as. a sludge. The acid with sludge was discardedcutside harbor limits.

(g) The bottom, top .and sides of the tank were rinsed with water. The pumps and piping were also. rinsed.

(h) All previously rusted surfaces were. examined and showed no evidence of deleterious attack. Rust was 97% detached.

Example II The following derusting method was carried out on heavily rusted pieces of steel that were placed in a first glass container. Pieces of aluminum alloys were placed in a, second container and pieces of. previously derusted steel were placed in a third container.

(a) The three containers were nearly filled with 0.50% by volume of 66 Baum sulfuric acid in fresh Water.

(b) The containers were allowed to stand undisturbed for 24 hours at a temperature of approximately 83 F.

(c) The initially rusted specimens in the first container were examined and found to be 100% derusted.

(d) The aluminum alloys were examined and found to have a sunface wastage loss of less than .0001-inch thickness which isnegligible for structural considerations.

(e) The initially derusted steel, was examined and found. to have a surface wastage loss of; approximately .003 -inch.thickness which is also negligible.

Example III The following derusting method was carried out. on rusted pieces of steel in a, glass container.

(a) Thecontainer was nearly filled with a fresh Water solution of hydrochloric. acid having a concentration of .18 normal.

(b) The container was allowed to stand for 30 hours at 76 F.

(c) The pieces of, steel were, examined and found to be 100% derusted with no deleterious attack on the metal.

Preferably sulfuric acid is used to form the pickling bath because of its availability and its excellent performance. However, other acids can be used to form the pickling bath andwehave found satisfactory those nonoxidizing mineral acids having a degree of ionization of about 0.3 or higher for a normal solution at 18 C., such, for example, as hydrochloric acid.

The concentration of acid in the pickling bath is an important factor and we have found preferable under many. conditions a concentration of about .18 normal although any concentrationfrom about .09 normal to about .36 normal is satisfactory. A concentrated above about .36 normal is unsatisfactory because there is a significant acid attack and possible perforation of the metal on which the rust is; formed. Thus in the, case of derusting a;cargo.or-. storagetank, acid concentration above about .36 normal causes significant acid attack and. pitting of the tank wall. On the other hand, if an acid concentration less than about .09 normal is used there may result incomplete descaling because of depletion of the acid to the point where it is ineffective and such depletion may occur well before the descaling is complete. Furthermore, if acid concentration is less than about .09 normal, there may, be; required an, inordinate amQuntof time to achieve even incompletederusting andthe-use of such'dilute acid is thereforeunsatisfactory.

Temperature of the pickling bath during derusting is another important factor and we have found that it is desirable to neither heat nor cool thebath but instead to allow it to become thermally stable at the temperature of the ambient atmosphere or tank. A, common temperature has been found to be about 60? under many conditions. Under normal conditions aboard. tanker ships, the temperature of a pickling bath in a cargo tank will stabilize ata temperature between about 40 F. and F. and this is the temperature range that we have discovered to be satisfactory.

Time span of the derusting is another important. =factor and we have found that soaking for a period of two, days is preferable under many conditions. However, under other conditions, a time span from about 1 day. to about 4 days has been found satisfactory. In any: event. the soaking is continued until derusting is complete and, this is signaled when the acid bath concentration dropsrapidly to a lower level depending on the amount of ferrous oxide present, after remaining of fairly constant strength during the first part of the soaking. Thisconcentration drop is usually a reduction in normalityof .01- to .03.

The characteristics ofthe rust tov be removed arean important determinant with respect to the variables; of concentration, temperature and time span of; soaking. Thus, for example, the porous structure of the rust appearstohave abearing upon the time span ofthe soaking; Although the mechanism of the derustingisnot clear, we hypothesize that the chemical mechanism in volved in the derustingperhaps, may be as follows: The cold dilute sulfuric, acid first comes in contact-with, the exposed ferric oxide layer of the rust but there: appears to be no significant chemical action. Becausethe; ferric oxide layer is .porous, the acid soaks through and penetrates the intermediate layer of. magnetite'wherep. again there appears to. be no significant chemical action; Be: cause the magnetite. layer is also porous, the. acid" soaks on through and finally penetrates the bonding layerof ferrousoxide or hydroxide where action. appears. to OC? cur. The acid in the pores thus contacts. the ferrous oxide at a series of points which are spaced: from each other because the pores are spaced from each other.- The acid at any point attacks the ferrous oxideinka grad+ ually. expanding area that spreads out from the point until it merges with areas of attack from otherpoints. When the ferrous oxide is completely dissolvedin this manner the bond is also dissolved sothat the ferric oxide and magnetite layers slough off and form a sludge.

In determining the variables of acid concentration; temperature and time span of soaking for removing rust ina particular case we have: found that the concentration of the acid bath in most cases is preferably about .18 normal so that this becoms a known constant. The temperature we have found'is usually self-determined. For example, in removing rust-from acargo tank of a tanker ship, the pickling bath formed in situ in the tank will usually stabilize at a particular temperature so that the temperature also thus becomes a known constant. The time span is then selected so that if the concentration is below about .18 normal or the mean temperature is below about 60 F. the time span is increased accord)- ingly above about two days. On the other hand, iffthe concentration is above about. .18 normal or the;,temp erature is aboverabout 607 F. the time span is decreased correspondingly, below about two days. Wehave found erably be. decreased.

The discovery that relatively thick layers such as inch of rust can be removed withoutsignificant chemical attack of the base surface is completely .new "and unexpected. The teaching of the prior art is exclusively to the effect that an acid bath capable of removing rust must necessarily be of an activity that will deleteriously attack the surface on which the rust is formed when such acid is in contact with the metal for several hours or conversely that an acid bath which will not attack the base surface during an exposure of several hours is necessarily of an activity inadequate to remove the rust. In particular, there has never in the prior art been a teaching that a relatively thick layer of rust can be removed by a solution of sulfuric acid with the concentration below .36 normal and the temperature below 100 F.

Another new and unexpected discovery was made in the relative rate of rust removal at the beginning as compared with the end of the treatment period. Derusting was slow at first but increased rapidly with time. This may have been caused by acid penetration of the ferric oxide and magnetite layers of rust with little dissolution. When the ferrous oxide layer was reached, it was quickly dissolved, followed by rapid sloughing ofi of the upper layers. This mechanism would account for the presence of considerable sludge, following derusting, but little or no sludge in the first few hours.

Another new and unexpected discovery was made that the strength of the acid bath remains substantially constant during most of the soaking period but at the end depletes very rapidly to a lower concentration level. Because the acid bath unexpectedly is not depleted substantially during the early part of the soaking we found that it is possible to use a more dilute acid than would have been expected for removing thick rust and use of such unexpectedly dilute acid has the advantage of insuring no significant acid attack of the metal surface underneath the rust.

A further new and unexpeted discovery was made in that the acid concentration is not limited by the thickness of the rust scale. The thickest rust scale produced in tanker corrosion was found to yield to .09 normal sulfuric acid. This may be explained by the hypothesized selective attack by the acid on the relatively thin adhesive layer of ferrous oxide or hydroxide.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

We claim:

1. The method for removing a coating of rust scale from the inner ferrous metal surface of a tank, which comprises filling said tank, to a level corresponding to the level below which scale removal is desired, and soaking said surface below such level with a solution of a mineral acid that has a degree of ionization of at least about 0.3 for a normal solution at 18 C., and an initial acid concentration sufficient to provide during the major period of such soaking an acid concentration within the range from about .09 normality to about .36 normality.

2. The method according to claim 1, wherein said acid is a non-oxidizing mineral acid.

3. The method according to claim 1, wherein said acid is sulfuric acid.

4. The method according to claim 1, wherein said solution is an aqueous solution containing sea water.

5. The method according to claim 1, wherein the acid solution has a temperature between about 40 F. and

(about .FJduring the major part of the soaking period. t

6. The method according to claim 1, and terminatin the soaking when the acid concentration remains fairly constant following a significant drop in concentration.

7. The method of removing adherent rust scale from the ferrous metal inner surface of a tank which comprises applying to such surface a dilute solution of a mineral acid that has a degree of ionization of at least about 0.3 for a normal solution at 18 C., in an acid concentration between about .09 normal and about 036 normal during at least the major part of the period of such application for a period necessary for removal of said rust scale from said surface.

8. The method according to claim 7, and terminating such application after a significant drop in the acid concentration in said applied solution.

9. The method according to claim 7, wherein the temperature of said acid solution in contact with said surface does not exceed about 100 F. during the major part of the period of said application of said solution to said surface.

10. The method according to claim 7, wherein said acid solution also contains a halide salt.

11. The method according to claim 7, wherein said solution is an aqueous solution containing a substantial amount of sea water.

12. The method of removing rust scale from a ferrdus metal surface without significant damage to such surface, which comprises applying to the rust scale on said surface a dilute solution of a mineral acid, that has a degree of ionization of at least about 0.3 for a normal solution at 18 C., and an initial acid concentration between about .09 normal and about .36 normal, until scale removal has been accomplished.

13. The method of removing rust scale from a ferrous metal surface without significant damage to such surface, which comprises applying to the rust scale on said surface a dilute solution of a mineral acid, that has a degree of ionization of at least about 0.3 for a normal solution at 18 C., and an initial acid concentration between about .09 normal and about .36 normal, and con tinuing such application of solution to said surface at least until the acid concentration has dropped to a substantially lower level from its initial concentration.

14. The method of removing from the ferrous metal surfaces of a tank interior, an adherent rust scale without causing significant damage to such metal surfaces, which comprises applying to such rust carrying surfaces, a dilute solution of a non-oxidizing mineral acid that has a degree of ionization of at least about 0.3 for a normal solution at 18 C. and having an initial concentraton of about .09 to about .36 normal, and terminating such application after the acid reaches a lower level of concentration after a significant drop in concentration.

15. The method of removing from the interior metal surfaces of a tank, an adherent rust scale without causing substantial damage to such metal surfaces, which comprises the steps of partially filling the said tank with water, adding to said tank a non-oxidizing mineral acid that has a degree of ionization of at least about 0.3 for a normal solution at 18 C., suflicient, when the tank is filled with additional water, to produce a concentration of said acid in said tank of about .09 to about .36 normal, filling the remainder of tank with additional water, retaining said dilute acid solution in said tank at ambient temperatures for a prolonged period at least until after such acid concentration drops substantially to a lower level.

16. The method of claim 12 in which the temperature of the acid solution is between about 40 F. and about 100 F. during the major part of the time of application.

17. The method of claim 13, in which the tempera- 7 8 ture'of the acid solution is between40 F. and 100 F. 1 2,410,322 Weesner Oct. 29, 1946 during the major part of the time of application. 2,878,146- Certa Mar. 17, 1959 References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS 5 American Machinist, vol. 70, N0. 12, Mar. 21, .1929,

1,225,956 Hofiman May' 15, 1917 pages 457-459 relied on. 1,373,573 Sorenson Apr. 5, 1921 Metallic Corosion Passivity and Protection, Evans, 1,770,712 Satler July 15, 1930 Edward Arnuld &-Co., London, 1948, pages-82-87 relied 2,049,517 Saukaitis Aug. 4, 1936 on. 

7. THE METHOD OF REMOVING ADHERENT RUST SCALE FROM THE FERROUS METAL INNER SURFACE OFA TANK WHICH COMPRISES APPLYING TO SUCH SURFACE A DILUTE SOLUTION OF A MINERAL ACID THAT HAS A DEGREE OF IONIZATION OF AT LEAST ABOUT 0.3 FOR A NORMAL SOLUTION AT 18*C., IN AN ACID CONCENTRATION BETWEEN ABOUT .09 NORMAL AND ABOUT 0.36 NORMAL DURING AT LEAST THE MAJOR PART OF THE PERIOD OF SUCH APPLICATION FOR A PERIOD NECESSARY FOR REMOVAL OF SAID RUST SCALE FROM SAID SURFACE. 